[PATCH v15 13/13] task_isolation self test

[Chris Metcalf]

This code tests various aspects of task_isolation.

Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
---
 tools/testing/selftests/Makefile                   |   1 +
 tools/testing/selftests/task_isolation/Makefile    |  11 +
 tools/testing/selftests/task_isolation/config      |   2 +
 tools/testing/selftests/task_isolation/isolation.c | 646 +++++++++++++++++++++
 4 files changed, 660 insertions(+)
 create mode 100644 tools/testing/selftests/task_isolation/Makefile
 create mode 100644 tools/testing/selftests/task_isolation/config
 create mode 100644 tools/testing/selftests/task_isolation/isolation.c

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index ff9e5f20a5a7..bd97479f44b3 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -23,6 +23,7 @@ TARGETS += sigaltstack
 TARGETS += size
 TARGETS += static_keys
 TARGETS += sysctl
+TARGETS += task_isolation
 ifneq (1, $(quicktest))
 TARGETS += timers
 endif
diff --git a/tools/testing/selftests/task_isolation/Makefile b/tools/testing/selftests/task_isolation/Makefile
new file mode 100644
index 000000000000..c8927994fe18
--- /dev/null
+++ b/tools/testing/selftests/task_isolation/Makefile
@@ -0,0 +1,11 @@
+CFLAGS += -O2 -g -W -Wall
+LDFLAGS += -pthread
+
+TEST_PROGS := isolation
+
+all: $(TEST_PROGS)
+
+include ../lib.mk
+
+clean:
+	$(RM) $(TEST_PROGS)
diff --git a/tools/testing/selftests/task_isolation/config b/tools/testing/selftests/task_isolation/config
new file mode 100644
index 000000000000..49e18e43b737
--- /dev/null
+++ b/tools/testing/selftests/task_isolation/config
@@ -0,0 +1,2 @@
+CONFIG_TASK_ISOLATION=y
+CONFIG_TASK_ISOLATION_ALL=y
diff --git a/tools/testing/selftests/task_isolation/isolation.c b/tools/testing/selftests/task_isolation/isolation.c
new file mode 100644
index 000000000000..d5f18231b94d
--- /dev/null
+++ b/tools/testing/selftests/task_isolation/isolation.c
@@ -0,1 +1,646 @@
+/*
+ * This test program tests the features of task isolation.
+ *
+ * - Makes sure enabling task isolation fails if you are unaffinitized
+ *   or on a non-task-isolation cpu.
+ *
+ * - Tests that /sys/devices/system/cpu/task_isolation works correctly.
+ *
+ * - Validates that various synchronous exceptions are fatal in isolation
+ *   mode:
+ *
+ *   * Page fault
+ *   * System call
+ *   * TLB invalidation from another thread [1]
+ *   * Unaligned access [2]
+ *
+ * - Tests that taking a user-defined signal for the above faults works.
+ *
+ * - Tests that isolation in "no signal" mode works as expected: you can
+ *   perform multiple system calls without a signal, and if another
+ *   process bumps you, you return to userspace without any extra jitter.
+ *
+ * [1] TLB invalidations do not cause IPIs on some platforms, e.g. arm64
+ * [2] Unaligned access only causes exceptions on some platforms, e.g. tile
+ *
+ *
+ * You must be running under a kernel configured with TASK_ISOLATION.
+ *
+ * You must either have configured with TASK_ISOLATION_ALL or else
+ * booted with an argument like "task_isolation=1-15" to enable some
+ * task-isolation cores.  If you get interrupts, you can also add
+ * the boot argument "task_isolation_debug" to learn more.
+ *
+ * NOTE: you must disable the code in tick_nohz_stop_sched_tick()
+ * that limits the tick delta to the maximum scheduler deferment
+ * by making it conditional not just on "!ts->inidle" but also
+ * on !test_thread_flag(TIF_TASK_ISOLATION).  This is around line 1292
+ * in kernel/time/tick-sched.c (as of kernel 4.7).
+ *
+ *
+ * To compile the test program, run "make".
+ *
+ * Run the program as "./isolation" and if you want to run the
+ * jitter-detection loop for longer than 10 giga-cycles, specify the
+ * number of giga-cycles to run it for as a command-line argument.
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <assert.h>
+#include <string.h>
+#include <errno.h>
+#include <sched.h>
+#include <pthread.h>
+#include <sys/wait.h>
+#include <sys/mman.h>
+#include <sys/time.h>
+#include <sys/prctl.h>
+#include "../kselftest.h"
+
+#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
+#define READ_ONCE(x) (*(volatile typeof(x) *)&(x))
+#define WRITE_ONCE(x, val) (*(volatile typeof(x) *)&(x) = (val))
+
+#ifndef PR_SET_TASK_ISOLATION   /* Not in system headers yet? */
+# define PR_SET_TASK_ISOLATION		48
+# define PR_GET_TASK_ISOLATION		49
+# define PR_TASK_ISOLATION_ENABLE	(1 << 0)
+# define PR_TASK_ISOLATION_USERSIG	(1 << 1)
+# define PR_TASK_ISOLATION_SET_SIG(sig)	(((sig) & 0x7f) << 8)
+# define PR_TASK_ISOLATION_GET_SIG(bits) (((bits) >> 8) & 0x7f)
+# define PR_TASK_ISOLATION_NOSIG \
+	(PR_TASK_ISOLATION_USERSIG | PR_TASK_ISOLATION_SET_SIG(0))
+#endif
+
+/* The cpu we are using for isolation tests. */
+static int task_isolation_cpu;
+
+/* Overall status, maintained as tests run. */
+static int exit_status = KSFT_PASS;
+
+/* Set affinity to a single cpu or die if trying to do so fails. */
+void set_my_cpu(int cpu)
+{
+	cpu_set_t set;
+	int rc;
+
+	CPU_ZERO(&set);
+	CPU_SET(cpu, &set);
+	rc = sched_setaffinity(0, sizeof(cpu_set_t), &set);
+	assert(rc == 0);
+}
+
+/*
+ * Run a child process in task isolation mode and report its status.
+ * The child does mlockall() and moves itself to the task isolation cpu.
+ * It then runs SETUP_FUNC (if specified), calls prctl(PR_SET_TASK_ISOLATION, )
+ * with FLAGS (if non-zero), and then invokes TEST_FUNC and exits
+ * with its status.
+ */
+static int run_test(void (*setup_func)(), int (*test_func)(), int flags)
+{
+	int pid, rc, status;
+
+	fflush(stdout);
+	pid = fork();
+	assert(pid >= 0);
+	if (pid != 0) {
+		/* In parent; wait for child and return its status. */
+		waitpid(pid, &status, 0);
+		return status;
+	}
+
+	/* In child. */
+	rc = mlockall(MCL_CURRENT);
+	assert(rc == 0);
+	set_my_cpu(task_isolation_cpu);
+	if (setup_func)
+		setup_func();
+	if (flags) {
+		do
+			rc = prctl(PR_SET_TASK_ISOLATION, flags);
+		while (rc != 0 && errno == EAGAIN);
+		if (rc != 0) {
+			printf("couldn't enable isolation (%d): FAIL\n", errno);
+			ksft_exit_fail();
+		}
+	}
+	rc = test_func();
+	exit(rc);
+}
+
+/*
+ * Run a test and ensure it is killed with SIGKILL by default,
+ * for whatever misdemeanor is committed in TEST_FUNC.
+ * Also test it with SIGUSR1 as well to make sure that works.
+ */
+static void test_killed(const char *testname, void (*setup_func)(),
+			int (*test_func)())
+{
+	int status;
+
+	status = run_test(setup_func, test_func, PR_TASK_ISOLATION_ENABLE);
+	if (WIFSIGNALED(status) && WTERMSIG(status) == SIGKILL) {
+		printf("%s: OK\n", testname);
+	} else {
+		printf("%s: FAIL (%#x)\n", testname, status);
+		exit_status = KSFT_FAIL;
+	}
+
+	status = run_test(setup_func, test_func,
+			  PR_TASK_ISOLATION_ENABLE | PR_TASK_ISOLATION_USERSIG |
+			  PR_TASK_ISOLATION_SET_SIG(SIGUSR1));
+	if (WIFSIGNALED(status) && WTERMSIG(status) == SIGUSR1) {
+		printf("%s (SIGUSR1): OK\n", testname);
+	} else {
+		printf("%s (SIGUSR1): FAIL (%#x)\n", testname, status);
+		exit_status = KSFT_FAIL;
+	}
+}
+
+/* Run a test and make sure it exits with success. */
+static void test_ok(const char *testname, void (*setup_func)(),
+		    int (*test_func)())
+{
+	int status;
+
+	status = run_test(setup_func, test_func, PR_TASK_ISOLATION_ENABLE);
+	if (status == KSFT_PASS) {
+		printf("%s: OK\n", testname);
+	} else {
+		printf("%s: FAIL (%#x)\n", testname, status);
+		exit_status = KSFT_FAIL;
+	}
+}
+
+/* Run a test with no signals and make sure it exits with success. */
+static void test_nosig(const char *testname, void (*setup_func)(),
+		       int (*test_func)())
+{
+	int status;
+
+	status = run_test(setup_func, test_func,
+			  PR_TASK_ISOLATION_ENABLE | PR_TASK_ISOLATION_NOSIG);
+	if (status == KSFT_PASS) {
+		printf("%s: OK\n", testname);
+	} else {
+		printf("%s: FAIL (%#x)\n", testname, status);
+		exit_status = KSFT_FAIL;
+	}
+}
+
+/* Mapping address passed from setup function to test function. */
+static char *fault_file_mapping;
+
+/* mmap() a file in so we can test touching an unmapped page. */
+static void setup_fault(void)
+{
+	char fault_file[] = "/tmp/isolation_XXXXXX";
+	int fd, rc;
+
+	fd = mkstemp(fault_file);
+	assert(fd >= 0);
+	rc = ftruncate(fd, getpagesize());
+	assert(rc == 0);
+	fault_file_mapping = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE,
+				  MAP_SHARED, fd, 0);
+	assert(fault_file_mapping != MAP_FAILED);
+	close(fd);
+	unlink(fault_file);
+}
+
+/* Now touch the unmapped page (and be killed). */
+static int do_fault(void)
+{
+	*fault_file_mapping = 1;
+	return KSFT_FAIL;
+}
+
+/* Make a syscall (and be killed). */
+static int do_syscall(void)
+{
+	write(STDOUT_FILENO, "goodbye, world\n", 13);
+	return KSFT_FAIL;
+}
+
+/* Turn isolation back off and don't be killed. */
+static int do_syscall_off(void)
+{
+	prctl(PR_SET_TASK_ISOLATION, 0);
+	write(STDOUT_FILENO, "==> hello, world\n", 17);
+	return KSFT_PASS;
+}
+
+/* If we're not getting a signal, make sure we can do multiple system calls. */
+static int do_syscall_multi(void)
+{
+	write(STDOUT_FILENO, "==> hello, world 1\n", 19);
+	write(STDOUT_FILENO, "==> hello, world 2\n", 19);
+	return KSFT_PASS;
+}
+
+#ifdef __aarch64__
+/* ARM64 uses tlbi instructions so doesn't need to interrupt the remote core. */
+static void test_munmap(void) {}
+#else
+
+/*
+ * Fork a thread that will munmap() after a short while.
+ * It will deliver a TLB flush to the task isolation core.
+ */
+
+static void *start_munmap(void *p)
+{
+	usleep(500000);   /* 0.5s */
+	munmap(p, getpagesize());
+	return 0;
+}
+
+static void setup_munmap(void)
+{
+	pthread_t thr;
+	void *p;
+	int rc;
+
+	/* First, go back to cpu 0 and allocate some memory. */
+	set_my_cpu(0);
+	p = mmap(0, getpagesize(), PROT_READ|PROT_WRITE,
+		 MAP_ANONYMOUS|MAP_POPULATE|MAP_PRIVATE, 0, 0);
+	assert(p != MAP_FAILED);
+
+	/*
+	 * Now fire up a thread that will wait half a second on cpu 0
+	 * and then munmap the mapping.
+	 */
+	rc = pthread_create(&thr, NULL, start_munmap, p);
+	assert(rc == 0);
+
+	/* Back to the task-isolation cpu. */
+	set_my_cpu(task_isolation_cpu);
+}
+
+/* Global variable to avoid the compiler outsmarting us. */
+int munmap_spin;
+
+static int do_munmap(void)
+{
+	while (munmap_spin < 1000000000)
+		WRITE_ONCE(munmap_spin, munmap_spin + 1);
+	return KSFT_FAIL;
+}
+
+static void test_munmap(void)
+{
+	test_killed("test_munmap", setup_munmap, do_munmap);
+}
+#endif
+
+#ifdef __tilegx__
+/*
+ * Make an unaligned access (and be killed).
+ * Only for tilegx, since other platforms don't do in-kernel fixups.
+ */
+static int
+do_unaligned(void)
+{
+	static int buf[2];
+	int *addr = (int *)((char *)buf + 1);
+
+	READ_ONCE(*addr);
+
+	asm("nop");
+	return KSFT_FAIL;
+}
+
+static void test_unaligned(void)
+{
+	test_killed("test_unaligned", NULL, do_unaligned);
+}
+#else
+static void test_unaligned(void) {}
+#endif
+
+/*
+ * Fork a process that will spin annoyingly on the same core
+ * for a second.  Since prctl() won't work if this task is actively
+ * running, we following this handshake sequence:
+ *
+ * 1. Child (in setup_quiesce, here) starts up, sets state 1 to let the
+ *    parent know it's running, and starts doing short sleeps waiting on a
+ *    state change.
+ * 2. Parent (in do_quiesce, below) starts up, spins waiting for state 1,
+ *    then spins waiting on prctl() to succeed.  At that point it is in
+ *    isolation mode and the child is completing its most recent sleep.
+ *    Now, as soon as the parent is scheduled out, it won't schedule back
+ *    in until the child stops spinning.
+ * 3. Child sees the state change to 2, sets it to 3, and starts spinning
+ *    waiting for a second to elapse, at which point it exits.
+ * 4. Parent spins waiting for the state to get to 3, then makes one
+ *    syscall.  This should take about a second even though the child
+ *    was spinning for a whole second after changing the state to 3.
+ */
+
+int *statep, *childstate;
+struct timeval quiesce_start, quiesce_end;
+int child_pid;
+
+static void setup_quiesce(void)
+{
+	struct timeval start, tv;
+	int rc;
+
+	/* First, go back to cpu 0 and allocate some shared memory. */
+	set_my_cpu(0);
+	statep = mmap(0, getpagesize(), PROT_READ|PROT_WRITE,
+		      MAP_ANONYMOUS|MAP_SHARED, 0, 0);
+	assert(statep != MAP_FAILED);
+	childstate = statep + 1;
+
+	gettimeofday(&quiesce_start, NULL);
+
+	/* Fork and fault in all memory in both. */
+	child_pid = fork();
+	assert(child_pid >= 0);
+	if (child_pid == 0)
+		*childstate = 1;
+	rc = mlockall(MCL_CURRENT);
+	assert(rc == 0);
+	if (child_pid != 0) {
+		set_my_cpu(task_isolation_cpu);
+		return;
+	}
+
+	/*
+	 * In child.  Wait until parent notifies us that it has completed
+	 * its prctl, then jump to its cpu and let it know.
+	 */
+	*childstate = 2;
+	while (READ_ONCE(*statep) == 0)
+		;
+	*childstate = 3;
+	set_my_cpu(task_isolation_cpu);
+	*statep = 2;
+	*childstate = 4;
+
+	/*
+	 * Now we are competing for the runqueue on task_isolation_cpu.
+	 * Spin for one second to ensure the parent gets caught in kernel space.
+	 */
+	gettimeofday(&start, NULL);
+	while (1) {
+		double time;
+
+		gettimeofday(&tv, NULL);
+		time = (tv.tv_sec - start.tv_sec) +
+			(tv.tv_usec - start.tv_usec) / 1000000.0;
+		if (time >= 0.5)
+			exit(0);
+	}
+}
+
+static int do_quiesce(void)
+{
+	double time;
+	int rc;
+
+	rc = prctl(PR_SET_TASK_ISOLATION,
+		   PR_TASK_ISOLATION_ENABLE | PR_TASK_ISOLATION_NOSIG);
+	if (rc != 0) {
+		prctl(PR_SET_TASK_ISOLATION, 0);
+		printf("prctl failed: rc %d", rc);
+		goto fail;
+	}
+	*statep = 1;
+
+	/* Wait for child to come disturb us. */
+	while (*statep == 1) {
+		gettimeofday(&quiesce_end, NULL);
+		time = (quiesce_end.tv_sec - quiesce_start.tv_sec) +
+			(quiesce_end.tv_usec - quiesce_start.tv_usec)/1000000.0;
+		if (time > 0.1 && *statep == 1)	{
+			char buf[100];
+
+			prctl(PR_SET_TASK_ISOLATION, 0);
+			printf("timed out at %gs in child migrate loop (%d)\n",
+			       time, *childstate);
+			sprintf(buf, "cat /proc/%d/stack", child_pid);
+			system(buf);
+			goto fail;
+		}
+	}
+	assert(*statep == 2);
+
+	/*
+	 * At this point the child is spinning, so any interrupt will keep us
+	 * in kernel space.  Make a syscall to make sure it happens at least
+	 * once during the second that the child is spinning.
+	 */
+	kill(0, 0);
+	gettimeofday(&quiesce_end, NULL);
+	prctl(PR_SET_TASK_ISOLATION, 0);
+	time = (quiesce_end.tv_sec - quiesce_start.tv_sec) +
+		(quiesce_end.tv_usec - quiesce_start.tv_usec) / 1000000.0;
+	if (time < 0.4 || time > 0.6) {
+		printf("expected 1s wait after quiesce: was %g\n", time);
+		goto fail;
+	}
+	kill(child_pid, SIGKILL);
+	return KSFT_PASS;
+
+fail:
+	kill(child_pid, SIGKILL);
+	return KSFT_FAIL;
+}
+
+#ifdef __tile__
+#include <arch/spr_def.h>
+#endif
+
+static inline unsigned long get_cycle_count(void)
+{
+#ifdef __x86_64__
+	unsigned int lower, upper;
+
+	asm volatile("rdtsc" : "=a"(lower), "=d"(upper));
+	return lower | ((unsigned long)upper << 32);
+#elif defined(__tile__)
+	return __insn_mfspr(SPR_CYCLE);
+#elif defined(__aarch64__)
+	unsigned long vtick;
+
+	asm volatile("mrs %0, cntvct_el0" : "=r" (vtick));
+	return vtick;
+#else
+#error Unsupported architecture
+#endif
+}
+
+/* Histogram of cycle counts up to HISTSIZE cycles. */
+#define HISTSIZE 500
+long hist[HISTSIZE];
+
+/* Information on loss of control of the cpu (more than HISTSIZE cycles). */
+struct jitter_info {
+	unsigned long at;      /* cycle of jitter event */
+	long cycles;           /* how long we lost the cpu for */
+};
+#define MAX_EVENTS 100
+struct jitter_info jitter[MAX_EVENTS];
+unsigned int count;            /* index into jitter[] */
+
+void jitter_summarize(void)
+{
+	unsigned int i;
+
+	printf("INFO: loop times:\n");
+	for (i = 0; i < HISTSIZE; ++i)
+		if (hist[i])
+			printf("  %d x %ld\n", i, hist[i]);
+
+	if (count)
+		printf("ERROR: jitter:\n");
+	for (i = 0; i < count; ++i)
+		printf("  %ld: %ld cycles\n", jitter[i].at, jitter[i].cycles);
+	if (count == ARRAY_SIZE(jitter))
+		printf("  ... more\n");
+}
+
+void jitter_handler(int sig)
+{
+	printf("\n");
+	if (sig == SIGUSR1) {
+		exit_status = KSFT_FAIL;
+		printf("ERROR: Program unexpectedly entered kernel.\n");
+	}
+	jitter_summarize();
+	exit(exit_status);
+}
+
+void test_jitter(unsigned long waitticks)
+{
+	unsigned long start, last, elapsed;
+	int rc;
+
+	printf("testing task isolation jitter for %ld ticks\n", waitticks);
+
+	signal(SIGINT, jitter_handler);
+	signal(SIGUSR1, jitter_handler);
+	set_my_cpu(task_isolation_cpu);
+	rc = mlockall(MCL_CURRENT);
+	assert(rc == 0);
+
+	do
+		rc = prctl(PR_SET_TASK_ISOLATION,
+			   PR_TASK_ISOLATION_ENABLE |
+			   PR_TASK_ISOLATION_USERSIG |
+			   PR_TASK_ISOLATION_SET_SIG(SIGUSR1));
+	while (rc != 0 && errno == EAGAIN);
+	if (rc != 0) {
+		printf("couldn't enable isolation (%d): FAIL\n", errno);
+		ksft_exit_fail();
+	}
+
+	last = start = get_cycle_count();
+	do {
+		unsigned long next = get_cycle_count();
+		unsigned long delta = next - last;
+
+		elapsed = next - start;
+		if (__builtin_expect(delta > HISTSIZE, 0)) {
+			exit_status = KSFT_FAIL;
+			if (count < ARRAY_SIZE(jitter)) {
+				jitter[count].cycles = delta;
+				jitter[count].at = elapsed;
+				WRITE_ONCE(count, count + 1);
+			}
+		} else {
+			hist[delta]++;
+		}
+		last = next;
+
+	} while (elapsed < waitticks);
+
+	prctl(PR_SET_TASK_ISOLATION, 0);
+	jitter_summarize();
+}
+
+int main(int argc, char **argv)
+{
+	/* How many billion ticks to wait after running the other tests? */
+	unsigned long waitticks;
+	char buf[100];
+	char *result, *end;
+	FILE *f;
+
+	if (argc == 1)
+		waitticks = 10;
+	else if (argc == 2)
+		waitticks = strtol(argv[1], NULL, 10);
+	else {
+		printf("syntax: isolation [gigaticks]\n");
+		ksft_exit_fail();
+	}
+	waitticks *= 1000000000;
+
+	/* Test that the /sys device is present and pick a cpu. */
+	f = fopen("/sys/devices/system/cpu/task_isolation", "r");
+	if (f == NULL) {
+		printf("/sys device: SKIP (%s)\n", strerror(errno));
+		ksft_exit_skip();
+	}
+	result = fgets(buf, sizeof(buf), f);
+	assert(result == buf);
+	fclose(f);
+	if (*buf == '\n') {
+		printf("No task_isolation cores configured.\n");
+		ksft_exit_skip();
+	}
+	task_isolation_cpu = strtol(buf, &end, 10);
+	assert(end != buf);
+	assert(*end == ',' || *end == '-' || *end == '\n');
+	assert(task_isolation_cpu >= 0);
+	printf("/sys device : OK (using task isolation cpu %d)\n",
+	       task_isolation_cpu);
+
+	/* Test to see if with no mask set, we fail. */
+	if (prctl(PR_SET_TASK_ISOLATION, PR_TASK_ISOLATION_ENABLE) == 0 ||
+	    errno != EINVAL) {
+		printf("prctl unaffinitized: FAIL\n");
+		exit_status = KSFT_FAIL;
+	} else {
+		printf("prctl unaffinitized: OK\n");
+	}
+
+	/* Or if affinitized to the wrong cpu. */
+	set_my_cpu(0);
+	if (prctl(PR_SET_TASK_ISOLATION, PR_TASK_ISOLATION_ENABLE) == 0 ||
+	    errno != EINVAL) {
+		printf("prctl on cpu 0: FAIL\n");
+		exit_status = KSFT_FAIL;
+	} else {
+		printf("prctl on cpu 0: OK\n");
+	}
+
+	/* Run the tests. */
+	test_killed("test_fault", setup_fault, do_fault);
+	test_killed("test_syscall", NULL, do_syscall);
+	test_munmap();
+	test_unaligned();
+	test_ok("test_off", NULL, do_syscall_off);
+	test_nosig("test_multi", NULL, do_syscall_multi);
+	test_nosig("test_quiesce", setup_quiesce, do_quiesce);
+
+	/* Exit failure if any test failed. */
+	if (exit_status != KSFT_PASS) {
+		printf("Skipping jitter testing due to test failures\n");
+		return exit_status;
+	}
+
+	test_jitter(waitticks);
+
+	return exit_status;
+}
-- 
2.7.2